What does caffeine, the most widely used psycho-active chemical in the world, do in the body?

As most college students who've studied late into the night can tell you, caffeine causes sleep inhibition while enhancing alertness and concentration. What they may not know, though, is that it also increases heart rate, causes the kidneys to increase urine production, and affects the blood vessels, dilating some and constricting others, and thus rerouting circulation to the body's various organs.